This invention relates to a detecting circuit for a photocell pattern sensing assembly and more particularly to such a detecting circuit for sequentially sampling an array of encoding photocells of a common type by sensing the combined output of one of the encoding photocells and the output of a reference circuit including an array of reference photocells of the same common type at a voltage comparator having a precisely controlled analog input signal threshold.
In photocell code pattern sensing assemblies an array of photocell sensors is arranged so as to have sensing positions corresponding to positions of binary coded segments of a code pattern. A specified or varying quantity of information is represented by the light transmitting or blocking states of the code pattern segments. The varying states of the segments are converted to binary coded electrical signals responsive to the outputs of the photocells Photocell signal output values are produced by known photoelectric effects of electron emission, generation of a voltage or by changes in electrical resistance depending upon the type of photocell. Accordingly, increases occur in the photocell current or voltage or decreases occur in the photocell resistance with increases in the light transmitted to the photocell. If light of a given high intensity and light of a substantially lower intensity are represented by two different binary states of the photocell outputs, at least one referenced photocell signal output magnitude must be established and detected as the photocell passes between the lighted and unlighted conditions.
It has been found that photocells of a given specified type may include photoelectric characteristics that vary between photocells of the common type. Accordingly, the photocell output values will vary for a given intensity of light. Also, where large numbers of photocell sensors are used in an array for sensing code pattern segments, there may be differences in the light intensities transmitted to different photocells when the several code pattern segments are in a given light passing or blocking condition. Therefore, another cause of variations in the photocell output values is incident light variations. The above undesired variations are sometimes cumulative and substantially increased when the photocell sensor arrays have large numbers of such photocells compactly mounted in close relationship within a small predetermined space. The small mounting spaces require that the photocell sensors have an extremely small size and close relationships so that there is some difficulty in shielding and isolating light intended to be transmitted to one photocell and blocked from an adjacent photocell by the segments of the code pattern. Also, mass production of photocell arrays by integrated and printed circuit techniques makes close tolerances of the photocell positions and the precise deposition of the photocell composition somewhat difficult. Some of the above-mentioned variable conditions are found in an optoelectronic utility meter register encoder assembly in which the present invention is utilized in one preferred embodiment.
One known technique for detecting a predetermined photocell signal output magnitude includes a Schmitt trigger detector in which the bistable circuit produces a constant amplitude output signal representing one binary state so long as the predetermined photocell output magnitude is exceeded. One such detecting circuit technique is suggested in U.S. Pat. No. 3,484,780 wherein a clipper and amplifier means or Schmitt is described directly connected to cell outputs to produce binary signals. It has been found that when such a bistable circuit is made for very low level signal operation that the bistable threshold level may vary during operation over an undesirable wide range, often as much as thirty to seventy percent. These wide threshold variations make it difficult to accommodate the above-mentioned variations in the photocell characteristics and light intensity variations so as to lead to inaccurate sensing of the code pattern. The inaccuracies further include deviations in the photocell signal output magnitude about the bistable circuit threshold level for a constant light transmitting state of a code segment so that the binary output does not stay stable or at a constant output during a given sampling time for a photocell.
Other detecting circuits for optoelectronic encoders are disclosed in U.S. Pat. Nos. 3,573,773; 3,609,726 and 3,815,126. The aforementioned U.S. Pat. No. 3,609,726 includes the use of photocell sensors in a meter register encoder having photocell outputs which provide signal variations to modulate a variable frequency detector signal and such a detecting technique is not used in the present invention. In U.S. Pat. No. 3,815,126 a meter shaft position encoder includes a photoresistive pattern array connected to a sensing circuit including FET solid state detecting devices. The deteching threshold of these devices is established by the lighted and unlighted resistance values of only the encoding photocell outputs. In U.S. Pat. No. 3,573,773 photocells of an optical pattern sensing array are connected in circuit networks or strings including resistors and diodes. The resistance of the photocells vary between one megohm and ten kilohm when unlighted and lighted, respectively. The outputs from the photocells are applied to an oscillator circuit which has a variable frequency corresponding to the number of lighted photocells.